(1) Field of the Invention
This invention relates to an improved aluminum-hydrogen peroxide battery which prevents the detrimental effects of peroxide on aluminum and eliminates the need for a resistive separator to separate anode or cathode compartments. More particularly, this invention relates to such a battery which utilizes a porous electrocatalyst composition to separate reactive catholyte solution from the anode compartment.
(2) Description of the Prior Art
Presently, a high power density primary battery based on aluminum and silver oxide alkaline half cells provide sufficient energy for vehicle propulsion. The major advantages of this electrochemical system is the extraordinarily high current densities, i.e., of the order of 1600 milliamps/cm.sup.2, which are readily achieved. The high current densities of the alkaline aluminum-silver oxide oxidation-reduction couple may be attributed to the anomalous solid phase mobility of the silver ion in the silver oxide cathode. Unlike other cations, the silver cation travels rapidly not only through the liquid phase but also through the solid phase of its salts. Therefore, as AgO is reduced Ag.sup.+ can continually travel to the electrode interface, preventing surface passivation and permitting continuous facile electron transfer. The major disadvantages of the alkaline aluminum-silver oxide primary battery are the significant costs of the silver oxide cathode and the relatively low faradic storage capacity of the silver oxide compared to aluminum.
In batteries which utilize a dissolved cathodic species, such as hydrogen peroxide, the dissolved cathodic species come into contact with an electrocatalytic electrode and are reduced during discharge. These solution phase batteries present problems which are not typically associated with solid cathode materials such as silver oxide. Specifically, unless isolated with a separator, the peroxide is in contact not only with the electrocatalytic electrode, but also with the solid anode. Peroxide contacting the aluminum anode can be detrimental towards battery performance in two ways. Firstly, the peroxide will "cathodically shift", diminish, the anode potential, and therefore reduce the battery voltage. Secondly, peroxide is chemically reactive with aluminum. Thus, the peroxide can create non-electrochemical energy-producing/"parasitic" chemical pathways for the consumption of reactive anode and cathode materials. When the rate of these chemical reactions is much slower than that of the electrochemical process, this parasitic reaction does not pose a problem. However, when the rate is comparable, a resistive separator is required to prevent the cathodic species from interacting with the anode to minimize the parasitic reactions. To provide a viable battery system, the electrochemical reactions must be maximized and the effects of the parasitic reactions minimized.
In the present state of the art, solution phase systems utilize a common chamber for the anolyte and the catholyte or two chambers that are separated by a resistive barrier. In the former case, the direct contact of the catholyte with the anode material causes voltage or material losses which adversely affect the battery performance. This results in either reduced efficiency or significant polarization losses. In the latter case, the resistive barrier creates an IR loss which reduces the net voltage expected from the system. As an example, U.S. Pat. No. 4,910,102 discloses a battery having an aluminum anode and a cathode wherein bipolar electrodes are interposed between the anode and the cathode. Means are provided for passing a hydrogen peroxide aqueous solution through the battery assembly. It is thus desirable to provide an aluminum based battery which permits substitution of the silver cathode in order to eliminate the disadvantages associated with the cathode.